A spectrum analyzer has been conventionally known that is adapted to remove an image frequency of an input signal by performing frequency sweep twice (for example, see Japanese Patent Laid-Open No. 08-233875, pp. 3-4, FIGS. 1-2). In this spectrum analyzer, two types of spectrum data are obtained by performing frequency sweep twice by changing the frequency range of an oscillation signal of a local oscillator to be inputted to an external mixer (an Nth order harmonic wave included in an output signal of the local oscillator is used as the oscillation signal), and image removal processing is performed by selecting a smaller data value corresponding to the same frequency.